The present invention relates generally to improved methods and systems for transmitting and receiving differential signals. More particularly, the present invention relates to methods and systems for transmitting and receiving differential signals over a plurality of conductors.
Signaling is the method by which information is communicated from one part of an electronic system to another. In single-ended signaling, a single conductor, such as a wire, is used to transmit data from a transmitter to a receiver. In binary single-ended signaling, a transmitter encodes a binary digit as a voltage value or a current value on the conductor. For example, in binary single-ended signaling systems commonly used in computer communications, a logical xe2x80x9c1xe2x80x9d may be indicated by a voltage of about +5 volts and a logical xe2x80x9c0xe2x80x9d may be indicated by a voltage of about 0 volts. A receiver converts the signal on the conductor into a binary digit by comparing a received voltage or current with a local reference voltage or current.
FIG. 1 is a circuit diagram illustrating a conventional binary single-ended signaling system. In this system, a transmitter 100 receives binary data and outputs a voltage signal on a conductor 102 based on the state of the binary data. For example, if the transmitter 100 receives a logical xe2x80x9c1xe2x80x9d, the transmitter 100 may output a signal of about +5V on the conductor 102. If the transmitter receives a logical xe2x80x9c0xe2x80x9d, the transmitter may output a signal of about 0V on the conductor 102. The two possible values, 0V and +5V, of the signal represent two possible symbols or data units of the binary single-ended signaling system. The symbols encode the data bits xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d. A receiver 104 comprises a comparator that converts the voltage on the conductor 102 to binary data by comparing the received signal to a reference voltage VREF. If the voltage of the received signal is greater than the voltage VREF, the receiver outputs a high voltage or logical xe2x80x9c1xe2x80x9d. If the voltage on the conductor 102 is less than the voltage VREF, the receiver 104 outputs a low voltage or logical xe2x80x9c0xe2x80x9d.
One problem associated with conventional single-ended signaling systems is that the requirement of a local reference voltage at the receiver results in increased power consumption. For example, the power required to transmit one bit in a single-ended signaling system includes not only the power required to send the signal over conductor 102, but also power required by a direct current voltage source to provide the reference voltage VREF at the receiver. Another problem associated with requiring a local reference at the receiver is that engineering a stable reference voltage may be difficult.
Another disadvantage associated with conventional single-ended signaling systems is that noise introduced in the signal transmitted over the conductor 102 may result in bit errors in the binary data output from the receiver 104. For example, the transmitter may encode a logical xe2x80x9c0xe2x80x9d as a low voltage signal on the transmission line. Additive noise may be introduced into the channel between the transmitter 100 and the receiver 104 and cause the low voltage signal to rise above the threshold voltage VREF. As a result, the receiver 104 may decode the signal as a logical xe2x80x9c1xe2x80x9d when a logical xe2x80x9c0xe2x80x9d was intended to be transmitted.
In order to alleviate the problems of requiring a local reference and bit errors caused by additive noise, differential signaling systems have been developed. In a differential signaling system, two conductors are oppositely driven to transmit a single signal. The receiver detects the difference between the voltages or currents on the two conductors. As a result, no local reference is required at the receiver. In addition, if the conductors are physically similar and positioned close to each other, noise introduced into the channel will be common to both conductors or common-mode noise. Receivers can be easily designed, e.g., using differential amplifiers, to cancel the common-mode noise.
FIG. 2 illustrates an example of a conventional differential signaling system. In FIG. 2, a transmitter 200 converts an input stream of binary data bits into a pair of equal and opposite currents at its outputs. The currents are transmitted through a pair of conductors 201 and 202 to a receiver 203. In order to transmit a logical xe2x80x9c1xe2x80x9d to the receiver 203, the transmitter 200 generates current flowing towards the receiver on the conductor 201 and an equal but opposite current flowing towards the transmitter 200 on the conductor 202. Similarly, in order to transmit a logical xe2x80x9c0xe2x80x9d, the transmitter generates a current flowing towards the transmitter on the conductor 201 and an equal but opposite current flowing towards the receiver 203 on the conductor 202.
In order to decode the received signals, the receiver 203 detects the voltage difference between ends 204 and 205 of a resistive terminator 206. If the voltage difference is positive, i.e., the voltage at end 204 is higher than the voltage at end 205, the receiver 203 may output a logical xe2x80x9c1xe2x80x9d. If the voltage difference is negative, i.e., the voltage at the end 204 is lower than the voltage at the end 205, the receiver 203 may output a logical xe2x80x9c0xe2x80x9d. In addition to converting the currents on the conductors 201 and 202 into voltages, the terminator 206 also reduces reflections on the conductors 201 and 202.
In addition to the advantages of operating without a local reference voltage and rejecting common-mode noise, differential signaling systems provide a larger signal swing for detection by the receiver than single-ended systems. For example, the voltage on each of the conductors 201 and 202 may swing from +V to xe2x88x92V, resulting in a voltage difference of 4V volts between symbols. Applying the same voltage ranging from xe2x88x92V volts to +V volts to a conductor of a single-ended signaling system results in a voltage difference of, at most, 2V volts difference between symbols. As a result of the increased voltage difference between symbols, differential signals have increased immunity to noise.
One disadvantage of differential signaling systems with respect to single-ended signaling systems is that conventional differential signaling systems require twice as many conductors as single-ended systems to transmit a given signal. That is, for a signal having M different signal levels or symbols, where M=2, one bit is transmitted per symbol in both single-ended and differential signaling systems. However, in single-ended signaling systems, the number of bits per symbol per conductor is 1; whereas, in differential signaling systems, the number of bits per symbol per conductor is 0.5. Thus, conventional two-conductor differential signaling systems are one-half as efficient as single-ended signaling systems with respect to the number of bits transmitted per symbol per conductor.
In order to improve the bits-per-conductor efficiency of differential signaling systems, pairs of differential signal conductors may be connected and driven to create additional channels per conductor. FIG. 3 illustrates one conventional method for improving the efficiency of a differential signaling channel. In FIG. 3, three differential channels X, Y, and Z are implemented using four conductors. Channels X and Y are conventional differential channels. The third channel, channel Z, is created without adding additional conductors. In the telephony industry, the third channel, channel Z, is referred to as a phantom channel.
In FIG. 3, channel X consists of a transmitter 300, conductors 301 and 302, a receiver 303, and center-tapped terminators 304 and 305. Channel Y consists of a transmitter 306, conductors 307 and 308, a receiver 309, and center-tapped terminators 310 and 311. In the illustrated circuit, the transmitter 300 transmits differential signals to the receiver 303 over the conductors 301 and 302, based on the digital input data XIn. Similarly, the transmitter 306 transmits differential signals to the receiver 309 over the conductors 307 and 308, based on the digital input data YIn.
The phantom channel, channel Z, is created by.driving the common-mode voltage of the X and Y channels. More particularly, a transmitter 312 is connected to the center taps of the terminators 305 and 311. A receiver 313 is connected to the center taps of the terminators 304 and 310. The transmitter 312 establishes a voltage difference between the center taps of the terminators 304 and 310 in response to the signal ZIn. The receiver 313 detects the voltage difference across the center taps of the terminators 304 and 310 and produces a digital output signal ZOut. The voltage difference generated by the transmitter 312 does not affect the signals XOut or YOut because the receivers 303 and 309 cancel the signal produced by the transmitter 312. The phantom channel creates an additional data path between the transmitters and receivers without requiring an additional conductor between the transmitters and receivers. Since the illustrated system is capable of transmitting three bits per symbol on four conductors, the efficiency is 0.75 bits per symbol per conductor.
FIG. 4 illustrates a well known conventional extension of the idea of driving the common-mode voltages of differential channels in which seven data channels are created using four pairs of differential conductors. In FIG. 4, channels X, Y, Xxe2x80x2, and Yxe2x80x2 are conventional differential channels, as previously described. Channels Z and Zxe2x80x2 are phantom channels, as described with respect to FIG. 3. A seventh channel, channel W, is created by driving and detecting the common-mode voltage of the phantom channels Z and Zxe2x80x2. The seventh channel is referred to in the telephony industry as a ghost channel. Because eight conductors are used to transmit seven symbols, each symbol encodes one bit, the efficiency of a communications system including a ghost channel is 7/8 bits per symbol per conductor.
The idea of driving the common-mode voltages of differential channels can be further extended to include a wraith channel. A wraith channel is created by driving the common-mode voltage of two ghost channels. In a system including a wraith channel, for two signal levels, 16 conductors transmit 15 symbols, and each symbol encodes one bit. Thus, the efficiency of a system using a wraith channel is 15/16 bits per symbol per conductor, which is near the efficiency of a single-ended signaling system.
One problem with the phantom/ghost/wraith channel hierarchy is that the absolute value of signals at a given receiver may be driven outside the common-mode range of the receiver. The common-mode range is the allowable voltage range at the input terminals of the receiver. In practice, the common-mode range may be limited such that the absolute voltage at either input terminal of the receiver never exceeds the voltage on the higher power supply terminal, nor falls below the voltage on the lower power supply terminal. Thus, even though phantom, ghost, and wraith channels improve the efficiency of differential signaling systems, they are limited by common-mode voltage range of the receiver. In addition, phantom, ghost, and wraith channels are unable to achieve the efficiency in bits per symbol per conductor of a single-ended signaling system.
One conventional method that may be used to improve the efficiency of any signaling system is to transmit more than one bit per symbol. In order to transmit more than one bit per symbol, more than two symbols for encoding data are required. One method of providing multiple symbols is to utilize multiple signal levels, where each signal level represents a different symbol. This technique is referred to as amplitude modulation. For example, a signaling system may include three possible signal levels: xe2x88x92V, 0, and +V, corresponding to three symbols. In a single-ended signaling system, an efficiency of log2(N), where N is the number of symbols, can be achieved. Therefore, for three symbols, an efficiency of log2(3) or about 1.5 bits per symbol per conductor can be achieved. The efficiency of a corresponding differential signaling system is 0.75. However, as discussed above, single-ended signaling systems require a local reference at the receiver, and when transmitting multiple signal levels, multiple references are required at the receiver. A conventional multi-level differential signaling system requires a receiver that compares the magnitude of the received signal to one or more references. Providing multiple receiver references increases power consumption and requires the engineering of stable DC sources.
Therefore, there exists a long-felt need for a differential signaling system having improved efficiency with respect to the number of bits transmitted per symbol per conductor, without requiring a local reference at the receiver.
Methods and systems for differential signaling according to the present invention result in increased efficiency in bits per symbol per conductor over conventional differential signaling systems. In order to achieve this increased efficiency, output signals from differential transmitters are summed and transmitted over multiple conductors to differential receivers. Summing the output signals results in multiple signal levels being transmitted to the receivers. However, unlike conventional multi-level signaling, no local reference is required at the receivers. The receivers need only detect the sign (positive or negative) of the received signal.
As used herein, the term xe2x80x9csymbolxe2x80x9d refers to a data unit transmitted or received during a single time period, such as a system clock period. The time period for transmitting or receiving a symbol is referred to herein as a symbol time. In a single-conductor signaling system, a symbol may be defined as a signal voltage or current level on the conductor. In a multi-conductor signaling system, a symbol may be defined as a combination of voltage or current levels on the conductors.
It is therefore the object of the present invention to provide methods and systems for transmitting and receiving differential signals over a plurality of conductors with improved efficiency in terms of the number of bits transmitted per symbol per conductor.
It is another object of the present invention to provide methods and systems for encoding and decoding binary data to be transmitted over a multi-conductor signaling system according to embodiments of the present invention.